Studies with isolated rat liver cells have demonstrated that lipid peroxidation in these intact cells is an ongoing process which, however, does not occur at a rate sufficient to alter the polyunsaturated fatty acid content of the cells during the incubation period in which they were studied (up to one hour). Prior treatment of the rats with phenobarbital resulted in essentially doubling of the rate of lipid peroxidation by isolated liver cells, but even under these conditions, homeostatic mechanisms appear to maintain a constant fatty acid composition of subcellular membranes. Exposure of the cells to CCl4 resulted in marked acceleration of lipid peroxidation which resulted in extensive alteration of the composition and content of fatty acids in the endoplasmic reticulum but not in the mitochondria. Exposure of the cells to NADPH plus Fe 3 ion resulted in vigorous peroxidative activity in the isolated liver cell which affected the fatty acid and protein content only of the plasma membrane-nuclei fraction. The studies demonstrate that peroxidative alterations may occur in different subcellular fractions depending upon the nature of the factors affecting the cells.